1. Field of the Invention
The present invention relates to a self-heat generation type honeycomb filter and its apparatus, and more specifically, to a honeycomb filter employed for the catalytic converter of an automobile in order to accelerate the activation of substance bringing about a catalytic action.
2. Description of the Prior Art
Conventionally, conversion from noxious elements such as CO, HC, and NOx, included in exhaust gas, to innoxious gas or water, has been performed, for example, through a catalytic converter provided in an exhaust pipe line from an engine for an automobile. However, there has been a problem which can not be solved simply by the use of the converter, that is, a catalytic substance is not substantially activated in a state that the temperature of exhaust gas from an engine just after it start up, is too low to purify the exhaust gas substantially.
Consequently, it has been proposed in the U.S. Pat. No. 3,770,389 and Japanese Patent Unexamined Publication No. 2-223622 that a catalytic converter comprising a self-heat generation type honeycomb carrier including a separate self-heat generation type honeycomb filter into which catalytic substance is supported, is provided to a honeycomb carrier into which the catalytic substance of a catalytic converter is supported, so that when an electric current is applied to the self-heat generation honeycomb carrier and it heats up, then the activation of the catalytic substance is accelerated.
This self-heat generation type honeycomb carrier is formed of a corrugated metal plate shaped in belt having a wave-like irregular surface successively bent and a plane metal plate shaped in belt having a flat surface, the corrugated and plane metal plates are put on another, and rolled together or laminated in layer.
A self-heat generation honeycomb carrier has been also proposed in that an electric current is applied from the center of a catalytic converter toward outer side face through electrodes provided at the center part and outer circumferential face to heat up the converter.
However, when such self-heat generation type honeycomb carrier is employed, a predetermined value of resistance is required for the corrugated and plane plates in order to electrically heat them up and raise the temperature. Consequently, when the carrier into which an electric current is applied from the central electrode toward the outer side face, is employed, a belt-like material for the corrugated and plane plates requires a substantial length for each to ensure the value of resistance.
On the other hand, such conventional honeycomb carrier having a sufficient metal foil portion in length, as mentioned above, increases itself in thermal capacity. Thus, another problem to be solved is created; a preferable high performance in purification can not be obtained unless sufficiently great electric current in magnitude is applied to the carrier, since an increase in temperature is relatively slow when an electric current is applied.
It is preferred that both catalytic parts of the corrugated and plane plates are mechanically joined, in order to provide a sufficient strength in structure against vibration from an engine. However, it is extremely difficult to join them, while the electric insulation between them is completely ensured. Especially, when an electric current is applied from the central electrode toward the outer side face, both end faces welded to join each other can not have a sufficient resistance. Further, the end faces of the honeycomb carrier can not be simply welded, so that it is very difficult to keep the electric insulation, while the sufficient strength is ensured.
It takes a long time to heat up the catalytic substance up to the activation temperature with the conventional catalytic converter, since, when the converter is electrically heated up, the heat raised by thermal conduction in the metal foil forming the corrugated and plane plates is diffused into the whole carrier.
As described above, it is extremely difficult to realize both requirements for the converter simultaneously; a preferred value in resistance and a decrease in thermal capacity.